The present invention relates to an electrolysis cell having bipolar elements for the electrolysis of, in particular, solutions of alkali salts.
Electrolysis cells having bipolar elements have been known for a long time. They have the advantage of a compact structure and thus a saving in space and a facilitated supply of electricity due to the fact that the unit cells are connected in series.
Nevertheless, despite their obvious advantages such cells are still relatively little used. This is largely due to the difficulties experienced in producing these cells. In actual fact, these cells consist of bipolar elements arranged side by side which are generally kept in place by clamping or securing the two end elements. However, this involves mechanical-type constraints, and, in particular, the clamping must be uniform and the elements must be very rigid and must not change shape during use.
A bipolar electrode for an electrolysis cell has been proposed in de Lachaux et al application, U.S. Ser. No. 478,605, filed June 12, 1974 now U.S. Pat. No. 3,980,545, characterized in that the cathode and/or anode frames are integral with the bimetallic base plate of the electrode, the said plate serving as a plane of reference and the perpendicularity with respect to this plane being ensured for the anode and cathode parts by the current leads to which they are welded. These current leads act as stiffeners and produce the planarity of these electrode parts and their parallelism with respect to the plane of reference constituted by the metal base plate. This planarity is a necessary condition for the good operation of such a cell, which involves being able to control the interpolar distance between the two electrodes.
However, this planarity condition is not sufficient by itself; it is also necessary that the interpolar distance be kept constant during the operation of the cell. Important progress has been made in this respect by using as electrodes metal structures of film-forming metals, such as titanium or metals and alloys of similar anode properties in the construction thereof, the anodically active parts of these structures being covered with conducting layers which are not attacked by the electrolyte employed.
Such structures have not only enabled the height of the electrodes to be increased on the one hand and the current density per unit surface area to be increased on the other hand without causing any excess heating, but have also made it possible to maintain the interpolar distance constant because these structures, contrary to what takes place with graphite anodes, do not alter their dimensions when in operation.
However, the improvement employed to meet these two requirements of planarity and constant thickness of the electrodes has been found to leave something to be desired.
As mentioned above, it is known that cells of the filter-press type or similar types generally consist of anode and cathode elements kept in place by clamping or securing the end elements. Since electrolytic solutions are particularly corrosive, very strict conditions of "tightness" or impermeability from frame to frame have to be observed.
The solution of the problem employed in similar cases is to arrange a sealing member between two adjacent rigid units such as described for example in Bouy et al U.S. Pat. No. 3,836,448. However, although this solution gives good results, it is not entirely satisfactory since with modern cells of large dimensions and high current densities, employed to provide a high yield, attempts are being made to maintain these cells in operation for as long as possible without any special maintenance. It is found that the sealing members have a tendency to age, which results in a shrinkage of the members, and as a consequence the interpolar distance varies despite all the improvements brought about in other respects.
It is an object therefore, of the present invention to provide an electrolytic cell free from the disadvantages of the prior art.
It is also an object of the present invention to provide an electrolytic cell in which the distances between bipolar electrodes are maintained without change.